Export & import storage archive (#726)

* core: import and export of chat archive

* export chat archive

* import archive, support starting chat after it is stopped

* test for maintenance mode

* test/fix archive with files

* prevent starting chat after chat database was deleted or imported

* update simplexmq

docs/rfcs/2022-06-03-portable-archive.md

@@ -0,0 +1,25 @@
+# Portable archive file format
+
+## Problems
+
+- database migration for notifications support
+- export and import of the database
+
+The first problem could have been solved in an ad hoc way, but it may cause data loss, so the proposal is to have migration performed via export/import steps.
+
+Out of scope of this doc - what will be the UX for database migration. It may be fully automatic, via code, with zero user interactions, or it could be via step by step wizard - irrespective of this choice it would include export and import steps.
+
+# Proposal
+
+Implement creating archive file and restoring from the archive in Haskell, application would only provide a source and target folders, respectively
+
+Archive files structure:
+
+- simplex_v1_chat.db
+- simplex_v1_agent.db
+- simplex_v1_files
+  - ...
+
+Archive file name (includes UTC time):
+
+simplex-chat.YYYY-MM-DDTHH:MM:SSZ.zip